This invention relates to communication systems for use in the extremely low frequency range (ELF) and, more particularly, to antenna systems for use with such systems. ELF communication systems operate at carrier frequencies which are generally between 10 Hz and 1000 Hz and frequently less than 150 Hz. This frequency range is recognized as being highly useful for large distance communication and for communication with submerged vessels.
Despite the acknowledged advantages of ELF communication, the weight and dimensions of suitable antenna systems have posed serious obstacles in planning for the storage and deployment of ELF antenna systems. This is particularly true where the antenna is to be carried by a vehicle or vessel.
ELF wavelengths are typically about 3000 km so that the antenna element of such antenna systems as horizontal and vertical dipoles must be impractically long in order to radiate efficiently. Previously proposed ELF communication systems have addressed the problem of deployment by proposing the burying of antenna elements or the use of utility lines as the antenna elements. Because vertical dipoles are preferable to horizontal antennas, owing to their more efficient radiation of power, the suspension of a vertical ELF dipole from a blimp or balloon has also been proposed. This latter concept is discussed in my U.S. Pat. No. 4,051,479 issued Sept. 27, 1977, the contents of which are hereby incorporated by reference. Although the use of such an antenna system is particularly attractive for mobile applications, a practical and severe limitation has been the weight of the conductor forming the antenna element, which is typically a metal wire. For convenience, the antenna element will be described as a wire although those skilled in the art will recognize that an antenna element of any cross-sectional shape is within the scope of the invention.
The weight of the antenna element limits the means by which the antenna can be lofted. It is highly desirable to use handloftable, rapidly deployable, inexpensive means such as balloons, kites, parafoils, and so forth. Each of the foregoing, however, has limited lifting capacity, particularly when one wishes to minimize the physical size of the deploying means in order to decrease the risk of detection, the storage size and/or, if applicable, the inflation time.
The weight of the antenna element can be reduced by minimizing the diameter of the antenna element. However, it has heretofore been believed by those skilled in the art that the wire diameter could not be reduced below a critical minimum, described more fully below, in order to avoid a coronal discharge at the operating voltage.
A vertical antenna is a high impedance device which accordingly requires high voltages between the wire and ground to obtain currents capable of producing useful radiated power. Wires which are thinner than the critical minimum diameter referred to above, cannot be subjected to high operating voltages without producing copious ionization in the vicinity of the wire surface. This phenomenon of space charge production is known as corona.
Corona occurs when the electric field near the antenna element causes the air to break down. Antennas have conventionally been operated below the corona inception voltage to avoid corona power loss and noisy currents in the antenna circuit. Stated alternatively, the diameter of the antenna element has always been relatively large to avoid corona at the operating voltage, since the strength of the electric field surrounding the antenna element is inversely proportional to wire diameter.
In the case of ELF frequencies where useful radiated signal strength necessarily requires long conducting wires, large diameters result in heavy conductors and substantial wind loading. Accordingly, large and/or expensive lofting vehicles are needed.